This works as designed. That code just seems to be there to ensure that the node type is actually available before attaching fields to it because errors would be thrown if not. If I had to make an educated guess, it needed to be added to blog_install() because the rest of the module's hooks, including blog_node_info(), probably weren't even invoked yet. So I doubt the node type would normally be available at that stage. It's probably less necessary for the forum module, since the body field is attached in forum_enable(). I could be wrong about all that, but the point is that since Poll doesn't have fields, it's not necessary.